Seasonal patterns of melatonin alter aggressive phenotypes of female Siberian hamsters

Many animal species exhibit year-round aggression, a behaviour that allows individuals to compete for limited resources in their environment (e.g., food and mates).
Interestingly, this high degree of territoriality persists during the non-breeding season, despite low levels of circulating gonadal steroids (i.e., testosterone [T] and oestradiol
[E2]). Our previous work suggests that the pineal hormone melatonin mediates a ‘seasonal switch’ from gonadal to adrenal regulation of aggression in Siberian hamsters
(Phodopus sungorus); solitary, seasonally breeding mammals that display increased aggression during the short, ‘winter-like’ days (SDs) of the non-breeding season. To test
the hypothesis that melatonin elevates non-breeding aggression by increasing circulating and neural steroid metabolism, we housed female hamsters in long days (LDs)
or SDs, administered them timed or mis-timed melatonin injections (mimic or do not mimic a SD-like signal, respectively), and measured aggression,...